When does hydrocephalus appear?

It varies. Hydrocephalus commonly called water on the brain can be present at birth or can develop later in life. It is due to an imbalance between production and absorbtion of brain fluid or csf. There are numerous causes of the congenital form. Other causes include infection and tumors. There is even a form seen later in life that mimics dementia and causes balance problems.
Hydrocephalus. Cause can be congenital (born with) or acquired from things like cancer or bleeding or a phenomena called psuedo ( sort of) hydrocephalus from obesity manifesting as headaches.